The present invention concerns television receivers having circuitry for driving a mechanical resonator such as a ceramic filter or the like, and more particularly, for driving such a mechanical resonator from a high impedance source such that the input impedance of the mechanical resonator substantially loads the source. As used herein, television receiver is intended to include any television signal processor such as a VCR or monitor, with or without a display device such as a CRT.
Ceramic filters and other mechanical resonating devices are commonly used in television receivers. For example, the video processing circuitry following a detector for providing a composite signal including color video and intercarrier components will often have a 4.5 MHz ceramic filter trap for removing the sound intercarrier signal from the composite signal produced by the video detector. Additionally, the sound processing circuitry will often have a 4.5 MHz bandpass ceramic filter for passing the intercarrier 4.5 MHz sound subcarrier while eliminating the rest of the combined signal. It is recommended by manufacturers of integrated circuits including detectors in applications notes that combined signals should be coupled to the ceramic filters by a low output impedance source such as an emitter follower. However, in order to improve performance, e.g., to improve signal to noise ratios, it is sometimes necessary to provide extra voltage gain not available from an emitter follower. One economical way to achieve this extra gain is to move one of the signal chains, e.g., the sound chain, to the collector electrode of the emitter follower. Such an arrangement is shown in U.S. Pat. No. 3,091,659 (Massman).
It is herein recognized that when the output for one of the signal chains, e.g., the sound chain, is moved to a transistor collector electrode, a problem arises because the output source impedance of the collector electrode of the transistor is high (typically more than 100 Kohms). This output source impedance is much more than, e.g., 10 times, the input impedance of the ceramic filter which varies considerably with frequency. For example, the ceramic filter sound bandpass filter of the exemplary embodiment has a relatively low input impedance, e.g., about 400 ohms at the 4.5 MHz center frequency, and a much higher input impedance, e.g., 2-3 Kohms, at frequencies removed from the center frequency. Since the relatively low input impedance of the ceramic filter loads the high impedance signal source, the gain of the transistor circuit at the collector electrode changes with frequency according to the change of input impedance of the ceramic filter with frequency.
This input impedance versus frequency characteristic of the ceramic filter is generally negligible when the signal is coupled from a low source impedance, e.g., 50 ohms or a source impedance comparable to the input impedance of the ceramic filter, e.g., that provided by an emitter follower. However, when driving the ceramic filter from a high source impendance, the input impedance versus frequency characteristic of the ceramic filter becomes an undesirable factor since it causes an increase in gain at frequencies, such as at the 3.58 MHz color subcarrier frequency, that the bandpass filter is intended to filter out. Accordingly, it is desirable to provide an economical way for providing extra gain by driving the mechanical resonant device such as a ceramic filter from the collector electrode of a transistor (rather than at the emitter electrode), while overcoming the input impedance loading effect of the ceramic filter on the high impedance signal source provided at the collector electrode.